Boron carbide/silicon carbide ceramics

ABSTRACT

Ceramic composites of silicon carbide (SiC) grains and boron carbide  (B.s4 C) grains which are uniformly coated with SiC are produced by reacting stoichiometric mixtures of silicon boride (SiB 4 , SiB 6 ) and carbon (graphite or carbon black) in situ.

This application is a division of application Ser. No. 09/005,823, filedJan. 12, 1998 now U.S. Pat. No. 5,894,066.

BACKGROUND

This invention relates to ceramics and more particularly to non-oxideceramics.

Boron carbide (B₄ C) has a high melting temperature, exceptionalhardness, and low specific gravity. However, boron carbide has a lowoxidation resistance and can not be used above 600° C. in an oxidizingatmosphere. Boron carbide also has a low toughness.

Introduction of silicon carbide (SiC) improves the oxidation resistanceof boron carbide ceramics. However, it would be desirable to provide B₄C/SiC ceramics which have greater oxidation resistance than materialspresently available in the art. Such improvements might be achieved by amore effective distribution of the SiC. However, it is desirable thatthese improvements be accomplished at little added cost.

SUMMARY

Accordingly an object of this invention is to provide a new B₄ C/SiCceramic material.

Another object of this invention is to provide a new B₄ C/SiC ceramicmaterial having improved oxidation resistance and toughness.

A further object of this invention is to provide a new method ofproducing a new B₄ C/SiC ceramic material.

These and other objects of this invention are achieved by providing aceramic composite comprising from about 64 to about 73 volume percent ofB₄ C and the remainder of the composite (from about 36 to about 27volume percent) being SiC, wherein the composite is in the form ofgrains of B₄ C which are uniformly coated with SiC and grains of SiCwhich are uniformly distributed among the SiC-coated B₄ C grains.

The ceramic composite is provided by heating a stoichiometric mixture ofa silicon boride powder that is SiB₄, SiB₆, or mixtures thereof withcarbon in the form of carbon black, a graphite powder, or mixtures thereof in an inert environment at a temperature of from about 1600° C. toabout 1850° C. until the silicon boride has reacted with the carbon toform a ceramic composite comprising B₄ C grains which are uniformlycoated with SiC and grains of SiC which are uniformly distributed amongthe SiC-coated B₄ C grains. Preferably the composite will then be hotpressed in the inert environment at a temperature of from about 1900° C.to about 2300° C. until it is fully densified.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of its attendantadvantages will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawing wherein:

FIGS. 1A, 1B, and 1C show microstructures for fully densified B₄ C/SiCceramic composites produced from (A) B₄ C and SiC powders, (B) SiB₄ andgraphite, and (C) SiB₄ and carbon black using the process of thisinvention; and

FIG. 2 is a high magnification scanning electron microscope picture of aB₄ C/SiC ceramic composite produced from SiB₄ and graphite using theprocess of this invention.

These FIGS. are discussed in the experimental section of the detaileddescription.

DETAILED DESCRIPTION

The present invention provides a ceramic composite comprising boroncarbide (B₄ C) grains which are uniformly coated with SiC and grains ofSiC which are uniformly distributed among the SiC-coated B₄ C grains.The ceramic composite is produced by reacting a silicon boride powderwith carbon powder in situ. The silicon boride used is silicontetraboride (SiB₄), silicon hexaboride (SiB₆), or mixtures thereof. Thecarbon powder is preferably carbon black, graphite powder, or mixturesthereof, with carbon black alone or graphite powder alone being morepreferred. The silicon boride powder and carbon powder are mixed in astoichiometric amount. The stoichiometric amount is calculated accordingto the following equations:

    SiB.sub.4 +2C→B.sub.4 C+SiC                         (1)

and

    SiB.sub.6 +5C→3B.sub.4 C+2SiC                       (2)

The reaction between SiB₄ and carbon produces a B₄ C/SiC ceramiccomposite composed of 64 volume percent B₄ C and 36 volume percent SiC,excluding porosity. The reaction between SiB₆ and carbon produces a B₄C/SiC ceramic composite composed of 73 volume percent B₄ C and 27 volumepercent SiC, excluding porosity. These volume percentages are calculatedfrom the weight ratios of the products (B₄ C, SiC) of the aboveequations (1) and (2) and the specific densities of B₄ C (2.520 g/cm³)and SiC (3.217 g/cm³). B₄ C/SiC composites over the range of from 64 to73 volume percent B₄ C with the remainder being SiC can be obtained byusing the appropriate mixture of SiB₄ and SiB₆ in the stoichiometricsilicon boride--carbon reaction mixture. Again the volume percentagesfor B₄ C and SiC are based on the volume of solid material (B₄ C andSiC) and excludes any porosity.

The first step of the process is a reaction step in which thestoichiometric silicon boride--carbon mixture is heated in an inertenvironment at a temperature of preferably from about 1600° C. to about1850° C., more preferably from 1750° C. to 1825° C., and most preferablyabout 1800° C. until the reaction between the silicon boride (SiB₄,SiB₆, or mixtures thereof) and carbon is completed. In the course of thechemical reaction, the formation of SiC, C, and B₁₂ (B,C,Si)₃, B₄ C--Sisolid solution were identified. As the reaction progresses withincreasing temperature or holding time, the silicon-enriched boroncarbide exsolves silicon converting gradually to stoichiometric B₄ C.The exsolved silicon, which is covering the surface of boron carbidegrains, then reacts with excess carbon forming uniform SiC coatings. Thefinal product is a ceramic composite comprising SiC-coated B₄ C grainsand SiC grains (formed early in the reaction process) uniformlydistributed among B₄ C grains. This reaction process is completed afterheating for 1 hour at 1800° C. The reaction process will take longer atlower temperatures, but the time required at a given temperature can beeasily determined. This reaction can be performed in pressurelessconditions or a pressure of from more than zero to about 10 MPa can beapplied during this step. The B₄ C/SiC composite produced pressurelesslyhas a porosity of about 25 percent. This composite structure can be usedas a ceramic preform which can be infiltrated by a suitable metal orceramic material.

The second step of the process is a hot pressing step which is used todensity the B₄ C/SiC ceramic composite produced in the first step above.Preferably the composite is fully densified in this step. In this stepthe B₄ C/SiC ceramic composite is hot pressed in an inert environment ata temperature of preferably from about 1900° C. to about 2300° C., morepreferably from 2000° C. to 2200° C., and still more preferably from2050 to 2150° C., and most preferably about 2100° C. Hot pressing the B₄C/SiC ceramic composites at 2100° C. and a pressure of 20 MPa for 0.5hour will produce a fully densified composite. A pressure of 20 MPa willalso work for temperatures in the range of from more than 2100° C. to2300° C. Greater pressure may be required for hot pressing attemperatures below 2100° C. For instance, a pressure of about 100 MPamay be required at 1900° C. The pressure required for hot pressing toachieve full densification at a given temperature can be easilydetermined by one of ordinary skill in the art using standardprocedures. A preferred range for the pressure in this step is fromabout 20 MPa to about 100 MPa. Finally, a fully densified B₄ C/SiCceramic composite will have a porosity of less than 2 percent.

The inert environment used in the reaction (first) step and thedensification (second) step may be a vacuum or a dry inert gas such asargon, helium, or neon. Care must be taken not to use a gas that willreact with any of the starting materials (SiB₄, SiB₆, carbon),intermediate reaction products, or final reaction products (B₄ C, SiC)at the high process temperatures.

The general nature of the invention having been set forth, the followingexamples are presented as specific illustrations thereof. It will beunderstood that the invention is not limited to these specific examplesbut is susceptible to various modifications that will be recognized byone of ordinary skill in the art.

Experimental

A two step hot pressing process comprising (1) a 1 hour hold at 1800° C.and 5 MPa to complete reactions and (2) a 0.5 hour hold at 2100° C. and20 MPa to density the material was applied to the following mixtures:(1) an equimolar mixture of B₄ C and SiC powders (as a control), (2) astoichiometric mixture of SiB₄ powder and graphite powder, (3) astoichiometric mixture of SiB₄ powder and carbon black, and (4) astoichiometric mixture of SiB₆ powder and graphite powder. FIG. 1A showsthe microstructure of the B₄ C/SiC ceramic composite produced from themixture of B₄ C and SiC powders (darker phase is B₄ C and lighter phaseis SiC). The composite has a fine-grained structure with uniformlydistributed, discrete B₄ C and SiC phases. FIG. 1B shows themicrostructure of the B₄ C/SiC ceramic composite produced from thestoichiometric mixture of SiB₄ powder and graphite powder. The B₄ C/SiCceramic composite produced has some SiC having a plate-like morphology(perpendicular to the pressing direction). FIG. 1C shows themicrostructure of the B₄ C/SiC ceramic composite produced from thestoichiometric mixture of SiB₄ powder and carbon black. This compositedoes not contain SiC exhibiting a plate-like morphology. However, thetortuous fracture surface of this composite material may indicate a veryhigh fracture toughness. Finally, the ceramic composite produced fromthe stoichiometric mixture of SiB₆ and graphite has a microstructure(not shown) similar to that produced from the stoichiometric mixture ofSiB₄ and graphite.

High magnification scanning electron microscope analysis of B₄ C/SiCceramic composites formed by reacting silicon boride (SiB₄ or SiB₆)powders with carbon (graphite or carbon black) indicated that SiC formsa continuous grain boundary phase on B₄ C. The phenomenon is most easilydetected in the B₄ C/SiC ceramics prepared by reacting the siliconborides with graphite and is clearly shown in FIG. 2 for ceramicsprepared from SiB₄ and graphite.

The preferred embodiment for a process is using a stoichiometric mixtureof a silicon boride (SiB⁴ or SiB⁶) and carbon and holding it in an inertenvironment at 1800° C. and 5 MPa for 1 hour and then holding it in theinert environment at 2100° C. and 20 MPa for 0.5 hour. Using SiB⁴ ispreferred where a higher SiC concentration is desired in the product forbetter oxidation resistance. The preferred embodiment compositecomprises 64 volume percent B₄ C and 36 volume percent SiC. Thepreferred embodiment composite is fully densified and has a porosity ofless then 2 percent.

Obviously, other modifications and variations of the present inventionmay be possible in light of the foregoing teachings. It is therefore tobe understood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically otherwise than asspecifically described.

What is claimed is:
 1. A ceramic composite comprising:from about 64 toabout 73 volume percent of B₄ C with the remainder of the compositebeing SiC, wherein the composite is in the form of grains of B₄ C whichare uniformly coated with SiC and grains of SiC which are uniformlydistributed among the SiC-coated B₄ C grains, and wherein the ceramiccomposite is fully densified.
 2. The ceramic composite of claim 1wherein the amount of B₄ C is about 64 volume percent.
 3. The ceramiccomposite of claim 1 wherein the amount of B₄ C is about 73 volumepercent.
 4. A ceramic composite comprising:from about 64 to about 73volume percent of B₄ C with the remainder of the composite being SiC,wherein the composite is in the form of grains of B₄ C which areuniformly coated with SiC and grains of SiC which are uniformlydistributed among the SiC-coated B₄ C grains, and wherein the ceramiccomposite has a porosity of from about 2 to about 25 percent.
 5. Theceramic composite of claim 4 wherein the amount of B₄ C is about 64volume percent.
 6. The ceramic composite of claim 4 wherein the amountof B₄ C is about 73 volume percent.